Google Maps(MULLINS, S.C.) -- Two female mental patients chained in the back of a county sheriff’s van drowned when the vehicle in which they were traveling was overcome by floodwaters, police said.

The deputies who were transporting the two women to a different facility in South Carolina were saved, local law enforcement officials told ABC News late Tuesday.

The two women have not yet been identified.

The pair of Horry County Sheriff’s deputies who were in the van tried to extricate the patients, but, due to rapidly rising floodwaters, were unable to open the van's doors to reach the shackled women, according to a statement from County Sheriff Phillip Thompson. Rescue teams responded in time to save the deputies.

The coroner in neighboring Marion County confirmed the two deceased patients were female, and the South Carolina State Law Enforcement Division is investigating the incident, according to the statement.

"Tonight's incident is a tragedy,” Thompson said in the statement. "Just like you, we have questions we want answered. We are fully cooperating with the State Law Enforcement Division to support their investigation of this event."

According to ABC affiliate WPDE, the patients were being transported from Loris Hospital Waccamaw Center for Mental Health to McLeod Health, which runs multiple facilities in the region.

The van was traveling west on Highway 76 into neighboring Marion County when floodwaters overcame the vehicle.

The incident happened in the area of the Little Pee Dee River, which branches off from the Lumber River, in Mullins, South Carolina. The Lumber River overflowed its banks following the record rains dumped by Hurricane Florence.

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